When a screen is updated by using a computer, scanning for an image display is performed in sequence from the top or bottom side or from the right or left side. As a result, an image is displayed scanning line by scanning line. In a computer used for common usage, an image update is performed a few dozen times for one second. The number of times of the image update for one second is referred to as a refresh rate. For example, when the refresh rate is 60 Hz, the image update is performed sixty times for one second.
As described above, when changeful images are displayed in real time on a display device which performs the image update a few dozen times for one second, an image is drawn at the same speed as that of the image update of the display device. Note that a time necessary for drawing an image of one screen is different depending on the number of displayed graphics. In other words, an image including a plurality of detailed graphics is drawn at a slow speed. If the drawing is not in time in an animation display, a drawing processing of a part of a graphic is cancelled, thereby performing real time display (Japanese Laid-open Patent Publication No. 11-296157).
Incidentally, as one computer control apparatus equipped with a display device, an electronic paper terminal device is used. In the electronic paper terminal device, a display technology using, for example, a cholesteric liquid crystal is used. The display technology using a cholesteric liquid crystal used in the above-described electronic paper terminal device has the advantage of being able to keep display contents even if the supply of power is cut off. Further, in a conventional electronic paper terminal device, sentences of books are intended to be read by using an electronic medium. For the purpose, a display image need not be updated in a frequency of a few dozen times for one second. In the electronic paper terminal device, the image update is performed, not at predetermined intervals, but when image data to be displayed is changed.
FIG. 20 illustrates a situation of an image update according to the conventional electronic paper terminal device. In an example of FIG. 20, there is illustrated a case of updating the display image from a state of displaying image data 91 including a graphic indicating a house to a state of displaying image data 92 including graphics indicating the house and a tree.
A first state (ST91) illustrates a screen 93 before an update. On this screen 93, only a graphic indicating the house is displayed. A second state (ST92) illustrates the screen 93 immediately after inputting an instruction of the screen update. When the instruction of the screen update is input, all the images displayed on the screen 93 are deleted. A third state (ST93) illustrates the screen 93 immediately after starting drawing the image on the basis of the image data 92. In this example, the image is displayed in sequence from the left side of the screen 93. A fourth state (ST94) illustrates the screen 93 on which a half of the images is displayed. At this moment, the drawing operation of a graphic of the newly-added tree is started. A fifth state (ST95) illustrates a state in which an image drawing is further advanced. A sixth state (ST96) illustrates a state after an update completion of the display image.
In an example of FIG. 20, the image is displayed in sequence from the left side to the right side, and on the contrary, the image may be displayed in sequence from the right side to the left side. Further, the image may be displayed downwards from the top side, or upwards from the bottom side.
As described above, in the electronic paper terminal device, when image data to be displayed is changed, the update of the display image is performed in sequence from the top or bottom side, or left or right side.
However, in the display technology used in the electronic paper terminal device, even if power is not supplied, an image can be kept, and on the other hand, a screen update speed is slow. Therefore, when the image is updated, a long time is necessary for grasping the whole image of the updated screen. When the electronic paper terminal device is specialized for reading a series of sentences as in a novel, a screen is displayed in sequence from the leading direction of sentences, thereby eliminating inconvenience for a user due to a delay of the screen update. Note that along with use expansion of the electronic paper terminal device, various contents need to be displayed. As a result, convenience of the electronic paper terminal device is impaired due to a delay of the screen update. As a multipurpose tool, the conventional electronic paper terminal device is not satisfactory for users in terms of operability.